isabella_ma,
I have rounded wooden fence posts that have lasted several years submerged in water. However, eventually wood rots. Even treated lumber will not sustain submerged conditions. Aluminum poles with pier augers are longer lasting. If you are bent on using wood, I would suggest reading up on how quick set concrete can be used for the base of your posts, as is done with building docks in lakes for boats.

Thanks deep roots,

I'll be using wood as it's in the upland portion of a wetlands, and retaining the natural feeling is a desired goal. I have gotten some furhter advice from my construction friends that concrete actually cures very well in saturated soils/groundwater. So I'll be putting in concrete piers with 4X4 metal post attachements on the top. The piers will be made using a sono-tube concrete form to prevent hole collapse in the saturated soils and to prevent phase separation of the cement components which may be caused by pouring them into standing water. The peirs will be constructed to be a few inches above ground level (in case of seasonal flooding), which will keep the post dry. These posts may actually last longer then the other posts installed above the groundwater level!!

What about using the metal bases that are pounded into the ground and then the post is attached on top. I have seen these at Lowe's. These are most often used for mail box posts, but may work for you.

I was looking at that set up, but I was concerned about the metal rusting out over the years. These fence is in an uplands area and depth to groundwater is only about I want to minimize the amount of disturbance to this environment, and to keep the ATVs from doing the same.

The engineer types at my office have said that concrete cures better in saturated soils than in dry soils.

A bit more expensive and time consuming, but within my skill-set.

Swamp cedar will last a little longer.

We have wood posts driven into the ground with metal spikes into our very wet yard, and I can confirm that they've been there for eight years or so, and they have really started to rust - some are almost completely rusted out. We thinking of redoing with concrete, too.

With a water table/marshy soil concrete is

an absolute and watch this video!

Here's a way to prevent decay for your fence, deck, pole barn, etc.